I have had Mirtazapine in addition to citalopram, but the Mirtazapine was at night to help with night time distress and SI.
My psychiatrist was wary of some combinations because of the risk of Serotonin Syndrome.

If you're on such a high dose of citalopram are you having an ECG done periodically as there is a small risk of heart rhythm problems with high doses of citalopram, I was advised to when on 40mg.

Best of luck with your trials, sorry I have no helpful suggestions.